1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, printer, facsimile device or a combination machine thereof that uses an electrophotographic system, and to a developing device and process cartridge installed therein, and more particularly relates to a developing device, process cartridge and image forming apparatus of a premix developing system that suitably supplies new carrier into the developing device.
2. Description of the Related Art
Disclosed in the past (for example, Japanese Patent Application Laid-open No. 2001-183893 (Prior Art 1) and Japanese Patent Application Laid-open No. 2000-112238 (Prior Art 2)) was technology (called a premix developing system) to suitably supplement new carrier into a developing device of an image forming apparatus such as a copier or a printer housing two-component developer comprising toner and carrier (this shall include developers to which additives are added).
In developing devices that use two-component developer, toner is suitably supplemented into the developing device from a toner supplement port provided on one part of the developing device corresponding to the consumption of toner in the developing device. The supplemental toner is agitated and mixed with the developer in the imaging apparatus by a transport member (agitating member) such as a transport screw. Part of the agitated and mixed developer is supplied to a developing roller. The developer supported on the developing roller is restricted to a suitable amount by a doctor blade, and then, in a position opposite a photosensitive drum, the toner in that two-component developer adheres to a latent image on the photosensitive drum.
In this way, carrier degrades over time because the carrier in the two-component developer housed in the developing device remains in the developing device without being consumed in the normal developing process. In more detail, this generates the “film scraping phenomenon”, in which the coating layer of the carrier is abraded or peeled off due to the carrier being agitated and mixed over a long time in the developing device causing a reduction in the charge capacity of the carrier, and the “spent phenomenon”, in which the toner components and additives adhere to the surface of the carrier causing a reduction in the charge capacity.
The premix developing system is for the purpose of preventing the reduction of the output image quality by this kind of carrier deterioration over time. Specifically, by suitably supplementing new carrier (or new two-component developer) into the developing device and suitably discharging some of the two-component developer housed in the developing device to outside of the developing device, the amount of deteriorated carrier in the developing device is reduced and the volume and charge capacity of the carrier housed in the developing device is maintained.
Image forming apparatuses that use this kind of premix developing system have more stable output image quality over time than do apparatuses that require replacement of the developing device and carrier with new products every time the carrier deteriorates over time.
Meanwhile, disclosed in Prior Art 1 and Prior Art 2, and the like, is an apparatus in a premix developing system that uses an overflow system as means for discharging developer to the outside of the developing device. In more detail, a discharge port (hole) is provided in a wall of the developing device, and when the developer surface that is carried to that location exceeds a specified height, that developer (developer that has become the surplus portion after supplementing carrier) is discharged to the outside from the discharge port.
Moreover, disclosed in Prior Art 2 and the like is a technology that, for the purpose of reducing the amount of developer discharged from the discharge port (developer discharge port), eliminates (or decreases the radius of) the screw part (blade) of the transport member (agitation and transport member) facing the discharge port.
In the developing device of the premix developing system of Prior Art 1 and the like described above, developer is added to the developer to be discharged until the necessary amount of developer has been discharged from the discharge port, and the amount of developer inside the developing device can become insufficient by discharging an excessive amount of the developer. Concretely, as opposed to only the developer that exceeds the specified height of the developer surface and that should be discharged from the discharge port, developer splashed by the transport member is also discharged from the discharge port.
When the amount of developer in the developing device becomes insufficient in this way, the deterioration status of the developer becomes unstable, the amount of static electricity of the toner drops, and problems occur such as an output image with decreased image concentration.
In order to resolve this kind of problem, the screw part of the transport member facing the discharge port was removed (or made a smaller diameter) in Prior Art 2. However, as a result of assiduous research, the three inventors of the present application discovered that it is not possible to fully suppress the problem of developer splashed by the transport member being discharged from the discharge port simply by removing the screw part of the transport member facing the discharge port.